I like Google Photos even though it has that limitation of maximum resolution of 16 megapixels (perfectly acceptable to me as most of my pictures are around 3-4 Megapixels only). If you can live with that limitation, Google Photos is free and best of all - unlimited cloud backup. Still, I will backup to external hard disk just in case.

Also, it's a convenient way to share out photos to the many chess players who ask me how or which website they can view their pictures ( a question I get a lot).

The apps for Google Photos on the iPad and Android is invaluable to me too because they automatically backup pictures to the cloud. After that I can erase all the pictures on the tablet and phone to free up space.